Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Full Text (PDF) References Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by LAY, T. Articles by WALLACE, T. C. Search for Related Content GeoRef GeoRef Citation

Multiple ScS attenuation and travel times beneath western North America

DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF MICHIGAN, Ann Arbor, Michigan 48109
DEPARTMENT OF GEOSCIENCES UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721

Abstract

Variations in attenuation and travel time of multiple ScS whole-mantle reverberations indicate very strong lateral gradients in upper-mantle properties beneath the tectonically active province of western North America. Whole-mantle attenuation estimates (Q_ScS) obtained by spectral stacking of phases sampling discrete subregions span the entire range previously determined using a similar procedure for a global set of continental and oceanic Q_ScS measurements. The relationship between travel time and attenuation anomalies, which has been used to argue for predominantly thermal control on the variations in oceanic environments, is poorly defined within the continental regime. However, a correlation between surface heat flow and Q_ScS variations in the United States indicates that thermal variations do control attenuation heterogeneity. For paths beneath Mexico, stacking 13 pairs of ScS_n phases from the 24 October 1980 Huajuapan, Mexico and the 19 June 1982 El Salvador events yields Q_ScS = 142 ± 10 in the frequency band 7.8 to 46.9 mHz. Time-domain measurements for the same phases yield a similar estimate of Q_ScS = 148 ± 9. The average ScS_n+1 - ScS_n JB (Jeffreys-Bullen) travel-time residual (t_ScS) is +0.9 sec (slower than JB). All of the differential travel-time estimates are corrected for dispersion effects in the passband of the observations, and are appropriate for a reference period of about 30 sec. Multiple ScS arrivals from the 29 April 1965 Puget Sound earthquake recorded by WWSSN, CSN, LRSM and Pasadena stations provide the first determination of whole-mantle attenuation beneath the westernmost United States. Pasadena recordings from three instruments provide ScS_n paths traversing the Cascade and Sierra Nevada ranges. Stacking 9 ScS_n pairs yields Q_ScS = 208 ± 39 (7.8 to 54.7 mHz) and the same phases give t_ScS = +2.5 sec. Six pairs of ScS_n reverberations under the Pacific Northwest give Q_ScS = 344 ± 88 (15.6 to 54.7 mHz) and t_ScS = +5.5 sec. A combined estimate for the relatively low heat flow region of the Pacific borderlands gives Q_ScS = 257 ± 20 (7.8 to 46.9 mHz), comparable to the high Q_ScS estimates previously determined for South America. On the other hand, 12 ScS_n pairs sampling the northern Basin and Range, which has high heat flow, yield a very low Q_ScS = 95 ± 4 (7.8 to 31.3 mHz) with t_ScS = +5.1 sec. The latter Q_ScS value is even lower than estimates for young oceanic regions, suggesting extensive partial melting in the low-velocity zone under the northern Basin and Range.